A distributed antenna system (DAS) means a system in which antennas are distributed and located at various locations and the antennas are managed by a single base station. In contrast, the conventional centralized antenna system (CAS) means a system in which the antennas of the base station are concentrated at the center of the cell.
The distributed antenna system may be distinguished from a femto cell/pico cell, a relay station, and an ad-hoc system. The distributed antenna system is different from the femto cell/pico cell forming a separate network by an individual antenna, in terms of the fact that all antenna areas are controlled by the central base station rather than that the area of the antenna itself is self-controlled by the individual antenna. The distributed antenna system may be distinguished from the relay station and the ad-hoc system in terms of the fact that each antenna can be connected by a wire. In addition, the distributed antenna system may be distinguished from a simple repeater in terms of the fact that each antenna transmits a different signal according to the command of the base station.
In early days, the distributed antenna system was provided to further install antennas within a cell in order to cover shaded areas and to transmit signals repeatedly. That is, the distributed antenna system was a main purpose of securing the coverage. However, when viewed macroscopically, the distributed antenna system may be considered as a kind of multiple input multiple output (MIMO) system in terms of the fact that the antennas can transmit or receive a plurality of data streams at the same time to support one or multiple users. The MIMO system has been recognized as a system that can satisfy the requirements of the next generation communications due to the high spectral efficiency. From the MIMO system viewpoint, the distributed antenna system has many advantages over the centralized antenna system. For example, there are advantages in that power efficiency can be high due to the reduction of the distance between the user and the antenna, the channel capacity can be high due to low correlation and interference between antennas, and communications with relatively uniform quality can be secured, regardless of the user's location within a cell.
In other words, the distributed antenna system having the above-mentioned advantages may be necessary to perform the MIMO transmission in order to secure high data capacity required in current and future communication standards. For example, it may be necessary to transmit rank 2 or more transmission to a single terminal (user equipment) (UE) in the same frequency domain (it is referred to as a single user MIMO (SU-MIMO)). In addition, the multi user (MU-MIMO) transmission that simultaneously supports several terminals may be required. This may be required in an uplink as well as a downlink.
The SU-MIMO and the MU-MIMO communications have been essentially considered in an IEEE 802 and a 3GPP LTE that are two standards organizations and actually covered in standards of an Institute of Electrical and Electronics Engineers (IEEE) 802.16e/m, 3GPP (3rd generation partnership project) LTE (long term evolution) Release 8/9, and the like. However, since the current communications standards were designed in consideration of the centralized antenna system (CAS), it is difficult to apply to the distributed antenna system (DAS) to which the advanced technology such as a MIMO technology is applied. Therefore, a communications standard that supports the distributed antenna system is required, and a data transmission method performed by a terminal that can be provided in the communication standard and the terminal that can be operated according to the method are required.